It is natural for the earth to be constantly changing. In Oregon, not too far from me, is the John Day Fossil Beds. 40 Million years of climate change are layered and documented there.
{short summary, longer description at the link below}
1) Clarno Formation (50 - 35 mya) evergreen tropical to subtropical forests. All the animal life in this layer is extinct.
2) John Day Formation (37 - 20 mya) A volcanic period with deciduous forests
3) Mascall Formation (15 - 12 mya) This is a warmer, wetter period which supported a lush mixture of hardwood forest and open savanna grassland.
4) Rattlesnake Formation (8 - 6 mya) a dry, cool climate that was dominated by grasslands. More info here:
http://en.wikipedia.org/wiki/John_Da...ional_Monument